The present invention relates to a hydraulic brake system with slip control for automotive vehicles comprising a master cylinder pressurizable by a hydraulic power booster, in which system valve means are inserted between the master cylinder and the wheel brakes connected to the master cylinder. The valve means allow removal of pressure fluid from the wheel brakes that can be replenished out of the pressure chamber of the hydraulic power booster by way of a change-over valve. A stroke limitation of the brake pedal is effected during slip control and the end surface of the master cylinder piston close to the working chamber is larger than the effective surface of the booster piston. A stepped piston is employed as the master cylinder piston, an annular surface thereof being adapted to be acted upon in the actuating direction by the pressure prevailing in the working chamber.
Such systems are known from German Pat. No. P3338249.2 which corresponds to copending U.S. patent application Ser. No. 660,469, filed Oct. 9, 1984. An essential component in the patent application is a braking pressure generator which is substantially composed of tandem master cylinder actuatable by a hydraulic power booster. For this purpose, the hydraulic power booster includes a brake valve by which a pressure is adjustable in the pressure chamber of the hydraulic power booster which is proportional to the actuating force respectively exerted on the brake pedal. As previously alluded to, wheel brakes are connected to the working chambers of the tandem master cylinder, one normally opened multi-directional control valve and one normally closed multi-directional control valve being allocated to each wheel brake. The normally opened multi-directional control valve is disposed in the flow path between the working chamber of the tandem master cylinder and the wheel brake, while pressure fluid is removed from the wheel brake through the normally closed multi-directional control valve if the normally opened multi-directional control valve was switched to its closed position by slip control electronics.
For supplying the hydraulic power booster with pressure, a hydropneumatic pressure accumulator is employed which is normally maintained at a predeterminable pressure level by a hydraulic pressure fluid pump and by corresponding pressure monitors. Connected to the pressure chamber of the hydraulic power booster is a three-way/two-position directional control valve which is adapted to be switched over by slip control electronics and which receives an electrical control command as soon as a critical slip condition occurs at one or more of the vehicle wheels allocated to the vehicle brakes. After the three-way/two-position directional control valve adapted to be activated by the slip control electronics has been switched over, the working chambers of the tandem master cylinder will communicate by way of sealing sleeves with the pressure chamber of the hydraulic power booster. This enables pressure fluid that was removed from the wheel brakes during slip control to be replenished out of the pressure chamber of the hydraulic booster.
Further components of the brake system described in the above-referenced patent application are two two-way/two-position directional control valves which are hydraulically actuatable and which will change their switch position as soon as there is changing-over of the three-way/two-position directional control valve connected to the pressure chamber of the hydraulic power booster. In this arangement, one of the hydraulically actuatable two-way/two-position directional control valves is connected to a chamber of the braking pressure generator with the chamber being confined by the housing and the master cylinder piston designed as a stepped piston. When the respective two-way/two-position directional control valve is changed over, the chamber referred to hereinabove will be shut off hydraulically. During control action, a hydraulic connection can be established by way of the other two-way/two-position directional control valve between a housing port of the tandem master cylinder and the unpressurized supply reservoir. As a result, during control action, the master cylinder piston designed as a stepped piston is caused to slide back in opposition to the actuating direction up to a predefinable axial position by virtue of pressurization of the master cylinder chambers.
As explained, pressurization of the wheel brakes during slip control takes place in each case dynamically by way of master cylinder sleeves inserted upstream of the master cylinder chambers. Alternatively the sleeves oppose a specific resistance to the volume flow of the brake fluid, and impair the proper function of the device--such as due to wrong assembly.
Therefore, it is an object of the present invention to simplify a hydraulic brake system with slip control of the type referred to and to increase its operational reliablity.